1998 wrangler code P0171 bank one lean jeep is getting 7 mpg black soot

yes code reader P0171 vehicle is currently at local dealer for third time in two weeks to diagnosis

What had to be done? Ive done the same things and have the exact same issues.

no one can pin point the problem so I installed a battery switch and every time the check engine light comes on I flip the switch and reset the computer now I average about 15-16mpg the light comes on every 100 or so miles it can be a pain on long trips but to go from 7mpg to 15 it is worth the trouble